Hi there!
Here are the important information we know:
Janie bought $15 worth of makeup
Janie spent $6 less than Leah spent
Steps to the answer:
If Janie spent $6 less than Leah, it also means that Leah spent $6 more than Janie.
If Janie spent $15 and Leah spent $6 more than Janie, you would have to add 6 to the amount that Janie spent :
$15 + $6 = $21
Your answer is: Leah spent $21 on makeup.
There you go! I really hope this helped, if there's anything just let me know! :)